Anal. Chem. 1987, 59, 213-217
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Remote Acquisition of Spectroelectrochemicai Data in a Room-Temperature Ionic Liquid with a Microprocessor-Controlled Fiber-optic Spectrophotometry System Edmund H. Ward and Charles L. Hussey*
Department of Chemistry, University of Mississippi, University, Mississippi 38677 Transmission spectroelectrochemistry, which employs optically transparent electrodes (OTE), is a well-established, expedient technique for the study of redox chemistry in aqueous and nonaqueous solvents (1-6). This technique has also been applied to molten salts, but on a less routine basis. Progress in this area has been reviewed (7). In our laboratory we are engaged in both electrochemical and spectroscopic studies of transition-metal and actinide redox chemistry (8-11) in anhydrous room-temperature haloaluminate molten salts (12, 13) like aluminum chloride-lmethyl-3-ethylimidazolium chloride (AlC1,-MEIC). Transmission spectroelectrochemical experiments have been undertaken to increase the scope of these investigations. Unlike most molten salts, these ionic liquids can be used as solvents at room temperature. However, the application of spectroelectrochemical techniques to these ionic liquids is not straightforward since these melts are extremely moisture and oxygen sensitive and can be manipulated only in a rigorously purified inert atmosphere or in sealed vessels. In addition, they react with many of the materials commonly employed in the construction of spectroelectrochemical cells, e.g., epoxy glue, Plexiglas, and most rubber compounds. Gold, which is often used to construct optically transparent electrodes, exhibits a limited positive potential range in these ionic liquids. Most spectroelectrochemical experiments with air-sensitive, reactive melts have been carried out in &-glass, vacuum-tight cells (14-19). There are several disadvantages to this approach. These fragile cells must be filled and sealed in a glovebox and then removed and installed in an external spectrophotometer in order to carry out an experiment. Once these cells are sealed, adjustment or replacement of the OTE is usually precluded, and the bulk solution in the cell cannot be replenished or changed without reopening the cell. In order to circumvent these impediments in our research, we have constructed a spectroelectrochemistry system that permits the acquisition of data directly inside a glovebox in a cell that is open to the glovebox atmosphere. This system consists of a microprocessor-controlled, fiber-optic spectrophotometer and a novel Teflon and glass cell suitable for use with a variety of solvents, provided that these solvents do not exhibit high vapor pressure. The application of fiber-optic technology to spectroelectrochemistry is not new since other workers have used fiber optics in conventional solvents to monitor either the electrode diffusion layer or the electrode surface (20,21) and to obtain absorption spectra in molten salts a t elevated temperatures during conventional bulk electrolysis experiments (22,23). However, we are not aware of any published reports in which fiber-optic waveguides have been used to completely isolate a cell with an OTE in a glovebox. In addition, no previous spectroelectrochemical studies have been reported in neat room-temperature haloaluminate ionic liquids.
EXPERIMENTAL SECTION Chemicals. The procedures used for the purification of AlCl,, chloride, and prepasynthesis of 1-methyl-3-ethylimidazolium ration of 49-51 mol % AlCl,-MEIC have been discussed in a previous publication (24). The synthesis of 1-methyl-3-ethylimidazolium hexachlororuthenate(1V) has been described (11). Instrumentation. The instrumentation used for spectroelectrochemistry consisted of a Guided Wave Model 100-2optical
waveguide spectrum analyzer, a Tandy Model 1000 personal computer, a Tandy Model F P 215 flatbed plotter, an AMEL Model 552 potentiostat, an AMEL Model 566 function generator, and a Houston Model 100 X-Y recorder or a Perkin-Elmer Model 56 Y-t recorder. The Guided Wave system was equipped with both tungsten and deuterium light sources, a silicon-diode detector, and a 1200 lines/mm concave holographic grating. The spectrophotometer was supplied with a proprietary interface card, which was designed for an IBM PC or compatible system, and a software package suitable for routine spectral measurements. Several minor modifications to this software were necessary in order to facilitate the present application. The Tandy computer was furnished with two disk drives, 128K of RAM on the mother board, and an RS-232 serial interface card. In addition, the computer was equipped with a TanPak 512K RAM expansion card (Hard Drive Specialist) for a total of 640K of RAM. This accessory board also included an additional serial communication port and a quartz clock. Part of the available memory was partitioned as a RAM disk to enhance the performance of the spectrophotometer control software. One serial port was connected to the plotter while the other provided a signal through a homemade digital relay that was used to trigger the function generator and chart recorder at the start of spectral acquisition. All experiments were conducted inside a Kewaunee Scientific Engineering Corp. glovebox system that was equipped with a Model 2C2500 30 ft3/min inert-gas purifier. The output from the spectrophotometer light source and the input to the monochromator were both directed to this glovebox with single-strand, fused-silica fiber-optic cables with poly(viny1 chloride) (PVC) coated monocoil jackets. These cables entered the glovebox through SMA fiber-opticcable bulkhead adapters that were sealed airtight into the glovebox wall. Both cables terminated inside the glovebox with adjustable-focal-length radiance probes containing collimating lenses. Cell and Cell Holder. The cell is depicted in Figure 1. The optical portion of the cell is a 1-mm-path-lengthquartz cuvette. The closed end of the cuvette was removed with a glass saw and then ground with 600-grit carbide paper and lapped on a glass plate with cerium oxide in order to obtain a smooth finish. The body of the cell was machined from a 7.5 cm X 1.5 cm X 1.5 cm block of Teflon. A seat for the end of the cuvette was cut into the center of this block with a small milling machine and was connected to the central passage just below it with a l-mm-diameter hole drilled in the center of the seat. It is important to keep this hole as small as practical since the solution trapped within contributes to the dead volume of the cell, i.e., solution that is not directly entrained within the porous electrode. The central horizontal passage in the lower block of Teflon was drilled slightly undersize in order to accommodate a 6-mm-long section of a 3-mm-diameter fritted glass rod as shown in Figure 1. This rod was obtained by sawing off the end of an ACE Glass porosity E (4-8 km) gas dispersion tube and was forced into the undersized passage in the Teflon block after the block was heated. Following this, the end of the passage was sealed with a threaded plug made of Teflon and holes were drilled for the 6-mm-diameter Pyrex counter and reference electrode tubes. The ends of these tubes were wrapped with Teflon tape and pressed into the block. The upper portion of the cell body was machined from a 7.5 cm X 1.5 cm X 0.7 cm block of Teflon. A seat was cut in this block to accommodate the top of the cuvette. A slot with dimensions slightly larger than the inside cross-sectional dimensions of the end of the cuvette was cut lengthwise through the block in the middle of this seat to permit the insertion of the reticulated vitreous carbon optically transparent electrode (RVC-OTE). The
0003-2700/87/0359-0213$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987 a 9
la
n
n
I C
..
A
E
I
h
.
f
d
/
top v i e w
f
I
C
I
=I-f
e Figure 1. Diagram of the RVEOTE spectroelectrochemical cell: (a)
counter and reference electrode compartments, (b) quartz cuvette containing the RVC slice, (c)brass clamping screw, (d) passageway connecting separator and RVC-OTE compartment (size exaggerated for clarity), (e)frmed glass separator, (f) aluminum plate, (9) lower cell body made of Teflon, (h) upper cell body made of Teflon. The slot for inserting the RVC slice through the top of f and h is not shown. aluminum plates shown in Figure 1 were used to provide rigidity when the upper and lower blocks were compressed in order to seal the bottom of the cuvette leak-tight against the lower seat. If the ends of the cuvette were sufficiently flat and smooth, this could be accomplished by tightening the clamping screws finger-tight, and no gasket was needed to prevent seepage. A slice of 100 ppi RVC (100 pores per linear inch, Energy Research and Generation, Inc., Oakland, CA) with cross-sectional dimensions approximating those of the inside of the cuvette served as the OTE. It was inserted through the top of the cell and completely filled the cuvette. The RVC slice was heated under vacuum at 500 "C for 1h, soaked in A1Cl3-MEIC melt overnight, washed with distilled water, and then dried at 100 "C in a vacuum oven prior to use. This pretreatment regimen was needed in order to reduce background currents associated with electroactive materials adsorbed on the RVC. The transmittance of a typical 1-mm-thick electrode was found to be approximately 35%, and like a neutral density filter, the transmittance was constant over the entire spectral range examined, ca. 330-700 nm. Electrical connections to the RVC-OTE were made with a micro alligator clip. A platinum-wire spiral served as the counter electrode. The reference electrode consisted of a 3-mm-diameter Pyrex tube with a small asbestos thread sealed in one end. This tube was filled with 66.7-33.3 mol % AlCl,-MEIC melt, and a short length of 1-mm-diameter A1 wire (Alfa, m5N) was immersed in this melt. This Al micro reference electrode was inserted into the cell through one of the vertical tubes. All potentials in the AlCl,-MEIC melt are referenced to this electrode. Experiments in aqueous solution were referenced to a saturated calomel electrode (SCE) and were performed outside the glovebox at a temperature of ca. 25 "C. Electrical connections to the RVC-OTE cell inside the glovebox were made with RG 58A/U coaxial cable. Cables for the working, counter, and reference electrodes were p d through the glovebox wall with airtight BNC coaxial cable bulkhead adapters. The cell holder (Figure 2) was machined from a 15 cm X 10 cm X 5 cm aluminum block. The block was drilled lengthwise to accommodate the 3.18-cm-diameter radiance probes and a 7.6 cm X 1.6 cm slot was cut completely through the center of the block for the cell. A mask with an 8-mm-diameter hole was placed in the slot between the cell and receiver probe in order to reduce stray light. The cell holder was heated by a 150-W Vulcan Electric cartridge heater inserted into a hole dried in the side of the block. The heater was powered by an ACE Glass temperature controller that was equipped with a thermistor sensor. The sensor was also
I I
I
e
side
I
IS
view e
Figure 2. Diagram of the spectroelectrochemical cell holder: (a) aluminum block, (b) adjustable collimating probes, (c) fiber-optic waveguides, (d) slot for spectroelectrochemical cell, (e) Teflon feet, (f) set screw, (9) thermistor probe, (h) Vulcan cartridge heater, (i)
thermocouple probe. implanted in the aluminum block. Procedure. A typical experiment with the molten salt was initiated inside the glovebox by assembling the cell and then weighing a few drops of solution containing the electroactive species into the OTE cavity with a microbalance. This was accomplished by placing the assembled cell on an analytical microbalance (Sartorius Model 1801MP8) inside the glovebox. The RVC slice was inserted into the OTE cavity and the cell sidearms were filled with pure melt to a level approximately equal to that of the melt in the RVC-OTE. If bubbles became trapped in the pores of the RVC after its insertion into the melt, they were removed by partially evacuating the cell in the glovebox antechamber. All spectral data were referenced to the spectrum of pure melt in the RVC-OTE. Electronic resistance compensation was applied during each experiment. The solution in the RVC-OTE could be changed by reassembling the cell with a new cuvette and RVC slice. The small amount of original solution remaining in the lower seat of the cell and the hole leading to the frit was blotted with a Kimwipe. It was not necessary to remove the melt in the counter or reference electrode compartments in order to effect this change. No carry-over from previous experiments could be detected when this procedure was performed.
RESULTS AND DISCUSSION Electrochemical Characterization of the BVC-OTE Cell in Aqueous Solution. A novel aspect of the cell design reported herein is the use of a fritted glass separator to isolate the analyte solution in the RVC-OTE. Edge effects should be eliminated or greatly decreased as a result of this arrangement because no part of the electrode is in direct contact with a reservoir of solution containing the electroactive species. Although this cell is intended for use in room-temperature ionic liquids (vide infra), electrochemical experiments were first conducted with 5.0 mM K,Fe(CN), in 1.0 M aqueous KC1. The [Fe(CN),]3-/[Fe(CN)6]4- redox couple has been used in the evaluation of numerous spectroelectrochemical cells, in-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
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1
0.2
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0
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-C
ElVl vs SCE
0.6
Flgure 3. Cyclic vottammetric current-potential curves for 5.0 mM K,[Fe(CNb] in 1.0 M aqueous KCI at scan rates of 1 mV/s (-) and 0.2 mV/s (--). The inRial potential was 0.50 V vs. SCE.
1 0
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I 10
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I 20
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I I 50
Time lrninl
Flgure 4. Double potential step chronocoulometric response for the reduction and oxidation of a solution that was'initially 5.0 mM K,[Fe(CN),] In 1 M aqueous KCI. The potential was stepped from an initial value of 0.40 V to 0.10 V and then returned to 0.40 V vs. SCE.
cluding a cell with a RVC-OTE (25),and electrochemical experiments conducted with this redox system provide comparative performance data. Cyclic voltammograms for 5.0 mM K,Fe(CN), in 1.0 M KC1 at scan rates of 0.2 and 1.0 mV/s, initiated from a potential of 0.50 V vs. SCE, are shown in Figure 3. The shapes of these current-potential curves are characteristic of those for a thin-layer cell in which the exhaustive electrolysis of an electroactive species confined at the working electrode takes place; i.e., the currents approach zero after passing through a maximum on the forward scan or a minimum on the reverse scan and the reduction and oxidation peaks are virtually identical in both height and area. The peak potential separations for these voltammograms are 70 and 160 mV, respectively, and they increased to much larger values at faster scan rates. It was possible to cycle between the two redox forms indefinitely. Double potential step chronocoulometricchargetime plots that resulted from stepping the potential of the RVC-OTE from 0.40 V to 0.10 V and then back to 0.40 V are shown in Figure 4. Inspection of these plots reveals that the charge becomes essentially constant after approximately 4-5 min of electrolysis. In addition, the charges associated with the reduction of [Fe(CN)# and oxidation of [Fe(CN),I4- are within k l % of one another (Figure 4).
0.4
0.2
0
-0.2
EIVI vs A I Flgure 5. Cyclic vottammetric current-potential curve for 2.09 mM [MEI],[RuCI,] In 49.0-51.0 mol % AICI,-MEIC at 40 "C. The initial potential was 0.60 V vs. AI, and the scan rate was 0.2 mV/s.
Taken together, the cyclic voltammetric and double potential step chronocoulometric results suggest that the edge effects associated with this RVC-OTE cell are very small and that the introduction of a porous barrier between the RVCOTE and the counter- and reference-electrodecompartments permits complete retention of the electroactive species in the RVC-OTE for at least 50 min without diffusional loss into the other compartments. This characteristic may be important when this cell is used to study slow coupled homogeneous chemical reactions similar to those associated with the oxidation of [WzC19]3-in AlCl,-MEIC (9). The time required for exhaustive electrolysis is comparable to that reported for an RVC-OTE of traditional design constructed from microscope slides (25). In addition, the voltammetric peak potential separations found here, which result from uncompensated iR drop, are no larger than those reported for the same solution at the same scan rate in a more traditional cell (25). Characterization of the RVC-OTE Spectroelectrochemistry System in a Room-Temperature Ionic Liquid. A RVC-OTE cyclic voltammogram of 2.09 mM [MEI],[RuC&] in the 49.0-51.0 mol % AlC1,-MEIC melt at 40 "C is shown in Figure 5. The [RuC@- chloro complex ion has been shown to undergo the reversible, uncomplicated electron-transfer reaction depicted in eq 1 at vitreous carbon in the AlCl,-MEIC [RuC16I2-
+ e- * [RUC16]3-
(1)
melt (21). The shape of this current-potential curve is very similar to that obtained for a thin-layer cell except for some distortion due to oxidation of Cl- in the melt at potentials above 0.50 V. The peak potential separation for the current-potential curve shown in Figure 5 is 86 mV at a sweep rate of 0.2 mV/s. This separation is somewhat larger than that found in 1 M KC1 at the same scan rate, and it suggests that the uncompensated iR drop associated with the melt is greater than that associated with the aqueous solution. This difference in iR drop probably reflects the smaller specific conductivity of the melt (0.029 mho/cm at 40 "C (26))relative to aqueous 1 M KC1 (0.104 mho/cm at 20 "C). The formal [RuC1613-redox reduction potential, E"', of the [RUC&]~-/ couple was estimated from the average peak potentials taken from the current-voltage curve shown in Figure 5 and was found to be 0.404 V. This value is in very good agreement with the estimate of 0.395 V obtained from voltammetric
216
ANALYTICAL CHEMISTRY, VOL. 59, NO. 1, JANUARY 1987
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1:
I"1
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Ldd--Ldo
OO
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Time(min)
Figure 6. Chronocoulometric (-)
and chronoabsorptometric (- -) responses for the reduction of 2.09 mM [MEI],[RuCI,] in 49.0-51.0 mol % AICI,-MEIC at 40 OC. The potential was stepped from 0.60 V to 0 V vs. AI, and the absorbance was monitored at 494 nm.
measurements at a rotating-disk electrode in the same melt at an identical temperature (11). Chronocoulometricand chronoabsorptometric experiments were conducted by stepping the potential of the RVC-OTE from 0.60 to 0 V vs. AI. Charge-time and absorbance-time plots resulting from these experiments are given in Figure 6. The charge-time plot was corrected for the background current in pure melt that arises from the reduction of chlorine generated at the RVC when it is held a t an initial potential of 0.60 V prior to the initiation of an experiment and from the reduction of other minor electroactive impurities present in the melt. The total charge accumulated and the number of moles of complex added to the RVC-OTE cell in each experiment were used to calculate n for the redox couple. The resulting value of n = 0.997 that was obtained from this calculation unequivocally demonstrates that this RVC-OTE cell can be used effectively without knowledge of the volume of the solution entrained in the RVC-OTE. The absorbance-time plot (Figure 6), which was obtained by monitoring the absorption maximum for [RuClG12-a t 494 nm (cf. Figure i'), is consistent with the chargetime plot in that it decreases as the charge-time plot increases and eventually becomes constant, indicating complete reduction of the [RuCl,$-. The residual absorbance shown in this plot arises from the reduced species. The comparatively long time required for exhaustive electrolysis in the AlCl,-MEIC melt relative to aqueous solution (vide supra) reflects the difference in the viscosities of the two solvent systems. The viscosity of the 49-51 mol % AlC1,-MEIC melt is approximately 19.9 CPat 25 "C (26). Consequently, the diffusion coefficient for a given electroactive species in this melt is only a small fraction of that found in solvents like water (0.90 CPat 25 "C). As a result, electrolysis currents are substantially smaller in the melt than in less viscous solvents, and much longer times are required in order to effect the complete oxidation or reduction of a given number of moles of an electroactive species. Absorption spectra were recorded after the application of steady-state potential steps. In these spectropotentiostatic is used to control experiments, the applied potential, Eappl, the ratio [Red]/ [Ox]. When Nernstian equilibrium has been established throughout the solution in the RVC-OTE at Eappl, this ratio is reflected by the absorption spectrum (5). It is possible to calculate E"' and n for the redox reaction from
Flgure 7. Absorption spectra for 2.09 mM [MEI]p[R~I,] in 49.0-51.0 mol % AICI,-MEIC at 40 OC at several applied potentials: (a) 0.600 V, (b) 0.475, (c) 0.450, (d) 0.425, (e) 0.400, (f) 0.375, (9) 0.350, (h) 0.
absorption spectra obtained at a series of Eappl with the following equation:
.Eappl= E"'
+ 2.3 RT/nF log [ARed - A ] / [ A- A,,,]
(2)
where A M , A, and Aox are the absorbance of the completely reduced solution, the absorbance of the solution at each inand the absorbance of the completely termediate value of Eappl, oxidized solution, respectively, at a suitable wavelength (5). Absorption spectra recorded at various Eapplare shown in Figure 7. The RVC-OTE was held at each potential until there was no further change in the optical response. Since edge effects are negligible in this cell, the current dropped to very small values when the optical response became constant. vs. log [ A M - A ] / [ A - A,,] derived from these A plot of ICapp] spectra gave a straight line with a slope of 0.059 V, corresponding to n = 0.95 at 40 O C , and a value of E"' = 0.410 V. These results are in good agreement with those obtained from the cyclic voltammetric and chronocoulometric experiments.
ACKNOWLEDGMENT The authors wish to express their appreciation to David A. LeFebre, President, Guided Wave, Inc., for providing uncompiled software, hardware, and technical assistance and to Kenneth R. Seddon, Sussex University, UK, for helpful discussions. Registry No. C, 7440-44-0; K,[Fe(CN),], 13746-66-2;AICl3, 7446-70-0; KC1, 7447-40-7; Fe(CN)64-,13408-63-4; RuClG3-, 21595-26-6; bis(1-methyl-3-ethylimidazolium)hexachlororuthenate(2-), 104947-70-8; 1-methyl-3-ethylimidazoliumchloride, 65039-09-0. LITERATURE CITED Kuwana, T. Ber. Bunsen-Ges. Phys. Chern. 1973, 7 7 , 858. Kuwana, T.; Winograd, N. I n Nectroanalyticai Chemistry; Bard, A. J., Ed.; Marcel Dekker; New York, 1974 Vol. 7, p 1. Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9 , 241. Heinernan, W. R. Anal. Chem. 1978, 50, 390A. Heineman, W.R.; Hawkridge, F. M.; Blount, H. N. I n €iectroana/yticai Chemistry;Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, P 1. Robinson, J. I n €/ectrochemistry-Specia/!.stPeriodical Reports ; Pletcher. D., Ed.; The Royal Society of Chemistry, Burllngton House: London, 1984; Vol. 9. Norvel, V. E.;Mamantov, G. I n Molten Salt Techniques; Lovering, D. G., Gale, R. J., Eds.; Plenum: New York, 1983; Vol. 1. p 151. Scheffler, T. B.; Hussey, C.L.; Seddon, K. R.; Kear, C. M.; Armitage, P. D. Inorg. Chem. 1983, 22, 2099. Scheffler, T. B.; Hussey, C. L. Inorg. Chem. 1984, 23, 1926. Hitchcock, P. B.; Mohammed. T. J.; Seddon, K. R.; Zora, J. A.; Hussey, C. L.; Ward, E. H. Inorg. Chim. Acta 1966, 113, L25. Hussey, C. L.; Appleby, D.; Hitchcock, P. B.; Seddon, K. R.; Zora, J. A.; Crisp, R. A. Inorg. Chern., In press. Chum, H. L., Osteryoung, R. A. I n Ionic Llquids; Inman, D., Lovering, D. G., Eds.; Plenum: New York, 1981; p 407. Hussey, C. L. I n Advances in Molten Salt Chemistry, Mamantov, G . , Ed.; Elsevier: Amsterdam, 1983; VOI. 5, p 185. Mamantov, G.; Norvell. V. E.; Klatt, L. J. Nectrochern. SOC. 1980, 127, 1768.
Anal. Chem. 1987, 59, 217-218 (15) S 0 r h M.; Smith, G. P.; Norvell, V. E.; Mamantov, G.; Klan, L. N. J . Electrochem. Soc. 1981, 128, 333. (16) Norvell, V. E.; Tanemoto, K.; Mamantov. G.; Klatt, L. N. J . Elecfrochem. SOC. 1981, 128, 1254. (17) Chapman, D. M.; Smith, 0 . P.; Sarlle, M.; Petrovic, C.; Mamantov, G. J . €lecfrmhem. SOC. 1984, 131, 1609. (18) brward, B. L.; Kian, L. N.; Mamantov, G. Anal. Chem. 1985, 57, 1773. (19) Chapman, D. M.; Buchanan. A. C.; 111; Smith, G. P.; Mamantov, G. J . Am. Chem. SOC. 1988, 108, 654. (20) Brewster, J. D.; Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560. (21) Pyun, C.-H.; Park, S.-M. Anal. Chem. 1988, 58, 251. (22) DeGuibert, A.; Plichon, V. J . Necfroanal. Chem. Inferfacial Necfrochem. 1978, 9 0 , 399.
217
(23) DeGuibert, A.; Plichon, V.; Badoz-Lambling, J. J . Nectroanal. Chem. InterfacialNecfrochem. 1979, 105, 143. (24) Wiikes, J. S.; Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263. (25) Norvell, V. E.; Mamantov, G. Anal. Chem. 1977, 49, 1470. (26) Fannln, A. A., Jr.; Floreani, D. A.; King, L. A.; Landers, J. S.; Plersma, B. J.; Stech, D. J.; Vaughn, R. L.; Wiikes, J. S.; Wlillams, J. L. J . Phys. Chem. 1984, 88, 2614.
RECEIVED for review July 7, 1986. Accepted September 8, 1986. This work was supported by the National Science Foundation, Grant No. CHE-8412730.
Preparation of Strongly Adherent Platinum Black Coatings C a r l A. M a r r e s e Sensor R&D Group, Bacharach, Inc., 625 Alpha Drive, Pittsburgh, Pennsylvania 15238 Platinized electrodes, in which a film of high surface area Pt black is formed, have found utility in many areas of chemistry. In our laboratories, we frequently employ platinized platinum wire for various applications in gas sensors (I). The advantage of platinized wires is the increased surface area of the Pt black coating. The high surface area improves electrical contact to gas diffusion electrodes, as well as improving the stability of reference potentials. Unfortunately, one of the problems encountered with Pt black coatings prepared from typical literature preparations (2)is the relative frailty of the Pt black matrix. Often we have noticed, during sensor operation, that the Pt black coating is completely absent from the wire after several weeks. In an effort to circumvent this loss of Pt black, we have sought a method to prepare platinized wire of high surface area and durability. One method of preparing durable platinum black coatings is to sinter the platinum particles of the coating after platinizing. However, this is an additional time-consuming procedure which can also decrease the surface area. We have found that platinizing with simulatneous ultrasonic agitation produces durable platinum black films of high surface area. The technique is quick (1-3 min) and, with no subsequent treatment, such as sintering, constitutes an advantage over present platinizing procedures. It was first introduced in 1938 by Cupr (3)who employed ultrasonic agitation to assist in the electroplating of copper. The technique has since then been used for tin and silver electrodepositions ( 4 ) and for the electroplating of smooth, bright platinum on copper (5). The platinum wires platinized with ultrasonic agitation for use in our laboratories were much more durable than nonultrasonically plated wires. In the strict Darwinian sense, we are platinizing by “natural selection”: Only those platinum particles that can endure the harsh ultrasonic agitation survive and become part of the electrode. EXPERIMENTAL S E C T I O N Equipment. Cyclic voltammograms were generated with an EG&G Princeton Applied Research Model 273 galvanostat/potentiostat. Recordings were made on a Hewlett-Packard Model 7044A x-y recorder. Electrochemical plating was achieved with a Hewlett-Packard Model 6202B dc power supply. The ultrasonic cleaner was purchased from Sonicor, Model SC-BOTH. The plating cell was a 50-mL beaker, whereas the voltammetric cell wa4 a conventional three-electrode cell with counter and reference compartments isolated from the working compartment by fine glass frits. Materials. Platinum electrodes were fabricated from 0.005 in. diameter wire (Sigmund Cohm Corp.) sealed in soft glass. The 0003-2700/87/0359-0217$01.50/0
exposed wire was cut to 5.0 mm yielding a geometric area of approximately 0.02 cm2.Electrical contact was made with mercury and copper wire. A Ag/AgCl (saturated KCl) reference electrode, with cracked bead tip, was employed. The counter electrode was braided Pt wire, coiled on a glass rod for support. The electrolyte, for area measurements, was 1M H2S04employing Fisher brand HPLC grade water. The platinic acid solution (1.4%PtCb2- and 0.02% Pb2+in dilute aqua regia) was prepared from 7.0 g of the 0.005 in. Pt wire in 100 mL of aqua regia (one part HN03, three parts HCl) diluted to 500 g of solution with the HPLC water. To this dilute solution, 0.1 g Pb(N03)2was added. (The chloride content was low enough to not cause the precipitation of PbC12.) K3Fe(CN)6was from Fisher Scientific and used without further purification. Platinizing Procedure. The plating apparatus, Figure 1, was constructed by cementing the 50-mL beaker to the bottom of the 1-L ultrasonic tub with Dow Corning Silastic 730 RTV [4], A braided, platinized Pt wire [6], serving as the counter electrode, was secured to the outside of the plating cell and connected to the positive pole of the dc power supply. The electrodes to be plated were immersed in the cell to an approximate depth of 0.25 in. [5], measured from the glass (electrode)tip. The temperature of the bath was 23 2 “C. To plate platinum, the ultrasonic cleaner was turned on, followed by the power supply, which was adjusted to 2 V. After the designated time, both power supply and cleaner were turned off and the electrodes were rinsed several times with the HPLC water. The areas were determined electrochemically by integration of the hydrogen adsorption processes in the cyclic voltammogram at 0.1 V/S.
*
R E S U L T S AND DISCUSSION Figure 2 illustrates the characteristic surface voltammetry of platinum in sulfuric acid electrolyte for ultrasonically platinized (A) and unplatinized (B) platinum wires (note current scales). The voltammograms clearly exhibit the hydrogen adsorption reductive processes (from +0.15 to -0.18 V), uncomplicated by impurity adsorption, such as Cl-. It is evident from the area under the H adsorption processes of the platinized wires that the technique produces a Pt black coating of high surface area. The small oxidation at +0.35 V is probably due to Pb, as P b is present in most electrodepositions of platinum (2). (The “Pb” oxidation is not present in the voltammogram for the unplatinized wire, and the peak current is linear with potential sweep rate and decreases with increasing time for soaking in concentrated HN03.) In every instance, wires that have been platinized with the aid of ultrasonic agitation yield electrochemical areas that are larger than those obtained without the agitation, Figure 3. As one can see, the ultrasonic method can yield platinized wires with areas as much as 6 times larger than their nonultrason0 1986 American Chemical Society
